JPS6256934B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a fog cooling method for a metal strip,
In particular, we have developed a method for efficiently cooling the metal strip while maintaining its good shape by adding a new method to blowing a mixture of gas and liquid droplets onto the metal strip after heat treatment. It is based on

Conventionally, gas jet cooling, water jet cooling, and the like have been generally employed as typical methods for cooling metal strips after heat treatment.

Gas jet cooling is a method of cooling by circulating reducing gas in the cooling zone and blowing it onto the metal band using a nozzle, but it is inefficient because it only utilizes the heat removal effect that relies exclusively on convection. The disadvantages are that the blower that circulates the gas consumes a lot of electricity, which increases costs, and that the cooling rate is slow.

On the other hand, water injection cooling injects water into the strip.
In particular, this method involves cooling while preventing the formation of a vapor film, but it has drawbacks such as the cooling rate being generally too fast and difficult to control, and the cost of reheating to the overaging temperature after rapid cooling.

By the way, cooling of the strip by fog or mist is of course basically known.
This method generates a spray that collides with the surface of the metal strip to cool it.
As seen in the disclosure of the above publication, the following problems remained in continuous heat treatment of metal strips, especially thin steel sheets.

(1) Cooling using a mixture of air and water uses the latent heat of vaporization of droplets to remove heat from the metal strip, so the cooling rate is naturally higher than that of gas cooling, and therefore it depends on the cooling conditions. However, the temperature distribution in the width direction of the strip tends to be non-uniform, which often results in defective shapes.In this regard, the relationship between the spray conditions of the air-water mixture and defective shapes is not always clear. However, it was not possible to put it into practical use.

(2) The configuration of a system that considers cooling using a mixture of air and water as a single system has not been finalized, and efficient recovery technology has not been established.

For this reason, attempts at fog cooling have failed to reach actual application for many years, but the inventors have investigated the essence of the problem and set out to establish this type of cooling technology. As a result of intensive research aimed at this goal, we have discovered the conditions for fog spray to maintain the shape of the strip in good condition, and succeeded in realizing an efficient fog spray cooling method that satisfies these spray conditions.

First, we studied the fog spray conditions necessary to uniformly cool the strip.

Now, if you immerse the hot strip in water,
It is generally known that a vapor film is generated non-uniformly on the surface of the strip, causing a temperature difference in the width direction of the strip, resulting in poor shape. Even in the case of cooling by fog spray, it was predicted that if the amount of liquid in the fog that collided with the strip was too large, a vapor film would be generated on the surface of the strip, which would deteriorate the shape of the strip.

Therefore, first, we generate fine water droplets with a diameter of 50 to 100 μm and collide them with the surface of the strip.
The condition of the fog and the shape of the strip were investigated as follows.

The ratio of the water droplet of the fog to the gas (air in the experiment) at the time of collision is the droplet space ratio f _l (=droplet volume/gas volume)
The results of investigating changes in the shape of the strip by varying the value of f _l in the range of 10 ^-2 to 10 ^-4 and varying the impact velocity v below 20 m/s are shown below. Shown in Figure 1.

The strip here is 0.7mm thick and about 1000mm wide.
The above fog spray was applied to both sides of the strip in a vertical furnace, and the shape after cooling was visually observed after cooling to about 700°C. In the figure, ◯ indicates good shape, △ indicates slightly defective, and × indicates defective.

According to this investigation, droplet void ratio f _l =10 ^-2 has little to do with impact velocity v, and the shape deteriorates and defects such as ear waves and warping occur, while f _l =10 ^-3
The shape is generally good, and there is no problem at all when f _l =10 ^-4 .

In addition, when f _l = 10 ^-3 , there was a tendency for the shape to collapse somewhat when the collision speed v was high, but this seems to be due to the fact that the cooling rate increased, making it easier to cause uneven cooling. It can be done.

After all, when the droplet void ratio f _l >10 ^-3 , there are too many droplets in the fog, and the liquid tends to stay on the strip surface, resulting in a non-uniform vapor film and non-uniform cooling. Seem.

In other words, it can be seen that the droplet void ratio at the time of fog collision needs to be in the range f _l ≦10 ^-3 .

Figure 2 shows the relationship between the droplet space ratio f _l and the heat transfer coefficient determined from the above sample strip temperature. As the temperature approaches 10 ^-3 from a low f _l , the heat transfer coefficient naturally increases and the cooling rate increases. is also getting bigger. In this case, the collision speed was kept constant at 10 m/s, but when f _l =10 ^- ₄ , the cooling rate of the steel plate strip with a thickness of 0.7 mm was approximately 220° C./s.

In addition, the thickness of the strip can be changed to 0.5mm in addition to 0.7mm.
We conducted a shape experiment using fog cooling as described above, varying the droplet size from 1.0 mm to 1.0 mm, but the results were almost the same as in the case of 0.7 mm. When the droplet space ratio f _l exceeds 10 ^-3 , shape defects often occur. Contrary to what happens, f _l ≦
It was confirmed that in the range of 10 ^-3 , no practical problems arise.

In addition, the tension of the strip during the observation experiment of shape defects was 0.5 to 3.0, which is the tension of a normal heat treatment furnace.
The tension was tested in the range of Kg/mm ² , but the tension had little to do with shape defects. Next, we investigated the droplet diameter of Fog.

When performing fog cooling in a vertical furnace, if the droplet diameter is large, the droplets will flow down due to gravity and collide with them, which will not only have a negative effect on the fog cooling effect but also make recovery difficult. .

When we actually performed fog cooling using water droplets in a vertical furnace, we found that if the droplet diameter exceeded 300 μm, the droplets would flow down into the collision area of the lower fog of the multi-stage spray, which would adversely affect the lower fog cooling effect. I understood.

Generally, when droplets present in a gas fall at a constant speed due to the action of gravity, the following equation (A) holds true.

U=1/18ρ _l −ρ/μgd ² ...(A) U: Sedimentation velocity [m/s] μ: Viscosity coefficient [Kgw/
m・s] g: Gravitational acceleration [m/s ² ] d: Droplet diameter [m] ρ _l : Droplet density [Kgw/m ³ ] ρ: Gas density [Kg
w/m ³ ] Substituting the experimental fog conditions into equation (A), ρ _l = 1000Kgw/m ³ (water) ρ = 1.2Kgw/m ³ (air) μ = 1.8×10 ^-5 Kgw/m・s (air) and d = 300 x 10 ^-6 m, then U in this state is U = 1/181000 - 1.2/1.8 x 10 ^-5 x9.
8×(300×10 ^-6 ) ² ≒2.7 m/s, and for HN gas (H ₂ 5%, N ₂ 95%), ρ=1.1Kgw/m ³ , μ=1.8×10 ^- ₅ Kgw/
m・s, so similarly, U=1/181000-1.1/1.8×10 ⁻⁵ ×9.
8×(300×10 ^-6 ) ² ≒2.7m/s is given.

In other words, if the sedimentation velocity exceeds approximately 2.7 m/s, it will adversely affect the cooling of the lower fog.

Here, the droplet diameter d, where the settling velocity of the droplet does not exceed 2.7m/s, is given by the following formula (B) d≦√(2.7×1.8×) {( _l −)× ……(B) It will be done.

Note that there is a distribution in the diameter of droplets discharged from the spray nozzle of the fog. On the other hand, cooling by fog spray is determined by the hydrodynamic field, so
It is necessary to adopt the volume average particle diameter defined by the following formula (C) as the droplet diameter d.

Here, n(x) is the number of droplets with flow diameter x.

As the fog conditions were determined experimentally as described above, we investigated a spray cooling system using fog.

First, there are methods for generating droplets using various nozzles.
It was found that both the fluid fine spray nozzle and the droplet diameter conditions were satisfied. Among these, in the two-fluid fine spray nozzle type, the gas supply pressure to the nozzle is high, and when cooling a wide range, the power to supply the gas may be too high. The one-fluid fine spray nozzle type is a method in which droplets are generated and the generated fog is made to collide with a strip using a blower with a large air volume.

It has become clear that according to this method, the wind pressure of the blower may be lower and the electric power may be lower than when using a two-fluid fine spray nozzle.

The recovery system is a circulation system that collects the fog directly from the side of the strip after spraying it onto the strip, then passes it through a cooler, etc. to separate the condensed liquid from the vapor, and then supplies only the gas to the blower again. By devising the system, a cooling system using fog was established in a vertical furnace that does not affect the upper and lower parts of the cooling zone, especially the lower part.

Note that since the amount of the liquid after separation is small, it may be discharged, or of course, after separation, it may be collected into a liquid tank and supplied from there to the nozzle again.

The configuration of the above-mentioned fog circulation system is shown in FIG. 3a. This figure shows only the upper row of one side of the strip.

1 After the fine droplets satisfying the above formulas (B) and (C) are generated by the single-fluid fine spray nozzle 3, the blast air sent to the manifold 4 by the blower 5 is used to fog the strip 1. Here, of course, several nozzles 3 are arranged across the width direction of the strip 1, so that the fog can be sprayed almost uniformly in the width direction of the strip 1. 2 in the figure is a deflector roll.

The droplet space ratio f _l is the amount of liquid from the nozzle and
Needless to say, it is necessary to maintain f _l ≦10 ^-3 by adjusting the air volume of the blower.

On the other hand, the fog sprayed on strip 1 is
The recovery chamber 6 installed outside the side wall of the vertical furnace 12 in the widthwise direction of the strip as shown in FIG. 4 collects the strip. is fed to the blower 5 again. The liquid droplets collected after condensation are sent to a liquid tank 10 through a pipe 8, and then supplied to the nozzle 3 again by a pump 9. Also, new liquid may be supplied from a new liquid tank 11 to the nozzle. FIG. 3b shows a system using a two-fluid fine spray nozzle 13, in which case gas is sent directly from the blower 15 to the nozzle 13. However, since a normal two-fluid fine atomizing nozzle requires a considerably large gas pressure, the blower 15 has a higher pressure than the blower 5 of FIG. 3a in the case of a one-fluid fine atomizing nozzle.

Although the case where each of the one-fluid fine-spray nozzle and the two-fluid fine-spray nozzle is used alone has been described above, it is of course possible to construct a metal band cooling system by using these in combination.

To explain one embodiment shown in Fig. 3a, one fluid fine spray nozzle 3 is installed per stage in the board width direction.
Arranged in 3 tiers, 4 pieces at 200mm pitch,
The blower 5 discharges the fog discharged from each of these nozzles.
In a specific system where strip 1 ^is sprayed with
200mm), and the discharge amount per nozzle is 0.
It is variable in the range of /min to 40/min. Also,
The blower 5 had a pressure of 100 mmA _q and a maximum air volume of 2000 m ³ /min. Using this system, we were able to achieve a maximum collision speed of about 20 m/s and a droplet void ratio of 10 ^-3 .

The droplet diameter d is determined when the droplet is generated by a one- or two-fluid fine spray nozzle, and is not affected by the wind pressure and air volume of the blower.

In addition, a one-fluid fine spray nozzle discharges liquid as a fine spray by applying pressure to the liquid, but at normal working pressure (1 to 5 kg/cm ² ), there is no significant change in the droplet diameter d. do not have.

However, since the amount of liquid ejected changes depending on the pressure, it is possible to change the droplet space ratio by controlling the nozzle pressure.

For example, one nozzle discharges at 10/min, sprays on a strip area of about ^0.5m2 , and the impact speed is 5m/min.
If the blower is used to adjust the droplet space ratio to be s, the droplet space ratio f _l is becomes.

As described above, according to the present invention, the application for cooling a thin metal plate that has been conventionally subjected to heat treatment is possible.
The use of fog, which has been considered impractical due to deterioration of its shape, is now possible because it satisfies simple conditions for droplet systems in conjunction with limiting the droplet space ratio. This can be realized without causing any shape defects to the strip, and economical fog cooling can be realistically achieved.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the influence of the impact velocity v of the fog colliding onto the strip and the droplet porosity f _l of the fog at the time of collision on the shape of the strip. 3a and 3b are explanatory diagrams showing different examples of the fog spray system, and FIG. 4 is an external view of the main parts of a specific example of the gas circulation system. 1... Strip, 3... 1 fluid spray nozzle,
5...Blower, 6,7,8,9...Recovery circulation system.

Claims (1)

[Claims] 1. When cooling a strip metal plate heated in a continuous heat treatment furnace or the like, the volume average droplet diameter d expressed by the following formula (1) is within a range that satisfies the following formula (2). Fog is generated in multiple stages along the direction of sheet passing using a one-fluid fine spray nozzle and/or a two-fluid fine spray nozzle, and the droplet space ratio based on the volume ratio of droplets and gas under the spray is fl≦10 ^{- 3.} A method for cooling a metal strip by spraying fog directly onto the surface of the strip metal plate. Record Here, n represents the number of droplets with diameter x. In the formula, d: Volume average droplet diameter (m) μ: Gas viscosity coefficient (Kgw/m・s) ρ: Liquid density (Kgw/m ³ ) ρ _l : Gas density (Kgw/m ³ ) g: Gravity Acceleration (m/s ² )